Abstract : The dielectric constants of oxygen, dry air, nitrogen and carbon dioxide were determined by measuring the resonant frequency shift that occurred when an evacuated microwave cavity was filled with each of these gases. This resonant frequency shift was measured by an electronic counter to within one KHz. The dielectric constant for each gas was determined at frequencies of 8.5, 9.0, 9.5 and 9.9 GHz. The effects of temperature and pressure were considered and the results were reduced to standard temperature and pressure. These results, slightly higher than previously published data, differed from previous results by less than 1.2%. (Author)